The present invention relates to a light-emitting device, particularly an electroluminescent device and displays incorporating such devices. The electroluminescence for the electroluminescent device may be provided by means of an organic light-emissive material (see for example International Publication WO90/13148 which describes electroluminescent semi-conductive conjugated polymers, such as PPV).
By way of background, FIG. 1 shows the typical cross-sectional structure of an organic light-emissive device. The device is fabricated on a substrate (1) coated with a transparent first electrode (2) such as indium-tin-oxide. The coated substrate is overcoated with at least one layer of a thin film of an electroluminescent organic material (3) and a final layer forming a second electrode (4) which is typically of metal. By using a transparent substrate (e.g. of glass or plastics material), light generated in the film (3) is able to leave the device by passing through the first electrode (2).
The performance of electroluminescent devices has advanced rapidly over the past few years. Due to their high efficiencies, the devices show potential for a wide range of display applications, from simple backlights to graphic displays, such as television screens, computer monitors and palm-top devices which may consist of several million pixels. However, there is considerable variation in the active lifetimes of red, green and blue organic electroluminescent systems, including polymer systems. For the purposes of the present specification, the active lifetime of an electroluminescent element is defined as the maximum time for which the element is able to produce at least a display-monitor level of brightness (for example, set at 100 cd/m2) when operating under a given drive scheme. For example, an electroluminescent device with a red light emitting polymer may have an active life of 30,000 hours at 5 volts, whereas a device with a blue light-emitting polymer may have an active life of only 1500 hours at the same voltage (see table 1).
The disparity in active lifetimes of organic light-emissive materials is significant because one factor in determining the useful life or service life of a graphic display incorporating such materials is governed by the shortest of the active lifetimes of the different polymers employed. (Another factor concerns decay rates causing colour shift, which can reduce overall colour purity, i.e. xe2x80x98whitexe2x80x99 becomes xe2x80x98off-whitexe2x80x99, and possibly also produce non-uniformity in the display). Accordingly, attempts have been made to improve the service life of graphic displays. For example, research has been conducted into upgrading the active lifetime of the xe2x80x98weak linkxe2x80x99 in such displays; namely, the relatively short-lived blue light-emitting polymers. Also, systems have been devised to compensate the device driving currentxe2x80x94either by using a sensing mechanism or by predicting the rate of performance decay in complex drive compensation electronicsxe2x80x94to maintain optimal performance with time. However, compensation mechanisms require complex and expensive circuitry which may also impose restrictions on the available aperture ratio.
An object of the present invention is to improve the service life of graphic displays incorporating organic light-emissive materials.
In accordance with a first aspect of the present invention, there is provided a light-emitting device comprising: a first electroluminescent element for emitting light of a first colour when energised; and a second electroluminescent element for emitting light of a second colour when energised, the first electroluminescent element having an active lifetime which is greater than that of the second electroluminescent element, characterised in that the second element is configured to operate at a lower brightness than the first element.
The brightness or luminescence of light emitted by an object may be measured in candela per square meter, and is a measure of the amount of light (number of photons) emitted per second per unit solid angle per unit area, as corrected for the sensitivity of the eye. The instantaneous brightness may varyxe2x80x94intentionally or otherwisexe2x80x94from one moment to another. When considering light-emitting devices for use in graphic displays, variations in instantaneous brightness may occur too rapidly to be detected by the human eye. Accordingly, the xe2x80x9cbrightnessxe2x80x9d which is of interest to the present invention is time-averaged to the extent necessary to smooth out localised or high speed variations in instantaneous brightness.
The first and second electroluminescent elements may comprise organic light emissive materials, and may be polymeric materials such as those discussed in WO90/13148 or WO92/03490.
The present applicant has appreciated the implication of the correlation between brightness and service life of devices employing organic light-emissive materials. The correlation is illustrated schematically in FIG. 2 for two electroluminescent elements employing different organic light-emissive materials, for example a red light emitter (R) and a blue light emitter (B) which have different active lifetimes. The correlation for each may be summarised as a relatively high brightness being indicative of a relatively short service life and vice versa. If both organic light-emissive materials are operated continuously at the same level of brightness, the material with the shortest active lifetime will fail (i.e. reach the end of its active life) first (in the example, the blue will fail before the red) and the device will be judged to have failed prematurely at t1. However if the material with the shortest active lifetime is operated continuously at a lower level of brightness than the other material, the service life of the device will be extended to t2.
The ratio between the brightness (B1) of the first electroluminescent element and the brightness (B2) of the second electroluminescent element may be substantially equal to the ratio between the active lifetime (xcfx841) of the first element and the active lifetime (xcfx842) of the second element (i.e. B1/B2=xcfx841/xcfx842) Suppose, for example, that there is an order of magnitude difference in the active lifetime of the two elements (e.g. active lifetime of the first element is 30,000 hrs and the active lifetime of the second element is 3,000 hrs). If the two elements are to fail at substantially the same time, it may be necessary to operate the second element at one tenth of the brightness of the first element.
There may be another advantage to operating the second element (shorter active lifetime) at a lower brightness than the first element. When operated continuously, the amount of light emitted per unit time by the first and, second elements may decrease or decay with time, with the rate of decay perhaps being greater for the second element. Thus, the perceived colour of the light-emitting device with both elements energised will drift with time because the contribution by the second element to the overall light output slowly decreases. However, by operating the second element at a lower brightness than the second element may have the effect of retarding the rate of decay in the amount of light emitted per unit time. In other words, the rates of decay in the amount of light emitted per unit time by the first and second elements may become more even. Hence, the problem of perceived colour drift with time may be alleviated.
The first and second electroluminescent elements may be energised by a common potential difference, for example by using a common cathode. The correlation between brightness (or luminance in cd/m2) and voltage is illustrated schematically in FIG. 3 for two electroluminescent elements employing different materials, for example a red light emitter (RED) and a blue light emitter (BLUE) which have different driving voltage characteristics. In fact, the red light emitter (RED) has a lower driving voltage characteristic than the blue light emitter (BLUE). Thus, by driving the two elements at a common potential (V1), the red light emitter will be operating at a higher brightness than the blue light emitter (B1 greater than B2). Accordingly, the goal of extending the service life of the device is attainable by operating at a common potential.
In an alternative embodiment, the first and second elements may be energised by different potentials. The second element may be energised at a higher potential than the first element. Referring to FIG. 3, driving the blue light emitter (BLUE) at V2 will yield a brightness B3 which is greater than B2. Of course, if performance characteristics were reversed in FIG. 3 or the RED and BLUE curves cross over in the drive potential regime, the first element may be energised at a higher potential than the second element.
The second element may be energised in pulses. Pulsing has the effect of lowering the time-averaged brightness as compared to operating continuously, e.g. at a constant potential. Thus, even though the blue light emitter (BLUE) is operating at a higher potential than before (V2 greater than V1), the service life of the device may still be greater than t1 because the emitter is only energised for a fraction (e.g. less than {fraction (1/10)}) of the overall time. The second element may be pulsed at a frequency in excess of 50 Hz, and perhaps at 100 Hz. Although lower frequencies would also have the effect of lowering the time-averaged brightness, it may be desirable in some applications to pulse at a rate which is faster than the eye response function. Each pulse may last for perhaps 200 microseconds, with perhaps 20 milliseconds between pulses.
The first and second elements may be pulsed, with the time-averaged brightness for the first element being greater than that for the second element. Thus, if the first and second elements are operating at a common potential, the first element may be energised for longer periods that the second element. This may be achieved by pulsing the first element more frequently than the second element or by increasing the duration of each pulse (pulse width) for the first element relative to that for the second element. Pulsing of both elements may be useful for light-emitting devices incorporated in passive matrix-driven displays.
The second element may be adapted to emit light over a larger area than the first element. The difference in light-emitting areas may be such that the total light output from the first element is substantially equal to that from the second element over a comparable time frame. For example, if there is no difference in duration of activation, the ratio between the light-emitting area (A1) of the first element and the light-emitting area (A2) of the second element may be substantially equal to the ratio between the brightness (B2) of the second element and the brightness (B1) of the first element (i.e. A1/A2≈B2/B1). By operating the second element at a lower time-averaged brightness than the first element, the amount of light observed by an observer from the elements will differ if the elements are of equal size (assuming the duration of activation is equal). This is because with normal eye constraints (e.g. sampling and resolution factors) the amount of light received from an element is related to the product of the time-averaged brightness and the area over which light is emitted. However, by increasing the size of the second element relative to the first element, such a difference may be offset somewhat or even totally compensated.
The brightnesses of the first and second electroluminescent elements may be chosen such that the half-lives of the first and second electroluminescent elements are substantially equal.
The choice of driving conditions for the two electroluminescent elements may be governed by material properties. FIG. 4 shows a plot of efficiency against current density for red and blue light-emissive polymers in the first and second electroluminescent elements, respectively. If the device is to operate at optimum efficiency, the first electroluminescent element will need to operate at a first current density "sgr"1, whilst the second electroluminescent element will need to operate at a second current density "sgr"2, with "sgr"1 greater than "sgr"2. The current density "sgr"2 may, for example, be achieved by operating the second element at a higher potential than the first element, and achieve a lower brightness by pulsing the second element.
The device may further comprise a third electroluminescent element for emitting light of a third colour when energised, the third electroluminescent element having an active lifetime in between that of the first and second electroluminescent elements. All elements may comprise organic light-emissive materials, and may be polymeric materials such as those disclosed in WO90/13148 or WO92/03490. All elements may be energised by a common potential difference, or at different potentials, perhaps with the second element at the highest potential and the first element at the lowest, and perhaps with the second element being pulsed.
The third element may be adapted to emit light over an area which is greater than the first element but smaller than the second element. The reason for doing this is again to achieve the desired total light output per unit time. The effect of doing this is the same as increasing the service life of the second element at the expense of the first element. This is because in practice, when dealing with a finite substrate, the light-emitting area of the third element will be normalised and the areas of the first and second elements will be relatively smaller and larger respectively.
There is also provided a graphic display incorporating a light-emitting device according to the first aspect of the invention, wherein each electroluminescent element in the device corresponds to a pixel for displaying graphic information.
In accordance with a second aspect of the present invention, there is provided a light-emitting device comprising: a first electroluminescent element for emitting light of a first brightness (B1) when energised by a predetermined potential; and a second electroluminescent element for emitting light of a second brightness (B2) when energised by the predetermined potential, the second brightness being less than the first brightness; characterised by means for energising the first and second electroluminescent elements at the predetermined potential, and in that the second electroluminescent element has a larger light-emitting area than the first electroluminescent element.
The second aspect of the present invention provides for increasing the light-emitting area of the element with the lowest brightness relative to that of the element with the highest brightness, with a view to reducing the difference in the total light emitted by the elements in unit time. In the absence of such an areal compensation, the brightest element (i.e. the first electroluminescent element) will emit more light than the other element when operating at a common potential.
The first or second electroluminescent element may comprise an organic light-emissive material. The organic light-emissive material may be polymeric.
The energising means may comprise at least one electrode common to both electroluminescent elements. For example, the energising means may comprise an anode and a cathode, each common to both electroluminescent elements.
The total light emitted by the first electroluminescent element may be substantially equal to the total light emitted by the second electroluminescent element in any given time interval when both electroluminescent elements are energised. The ratio of the first brightness (B1) to the second brightness (B2) may be substantially equal to the ratio of the light-emitting area of the second electroluminescent element (A2) to the light-emitting area of the first electroluminescent element (A1), i.e. B1/B2≈A2/A1.
The first and second electroluminescent elements may emit light of first and second colours respectively when energised, the first and second colours being selected from the group consisting of red, green and blue.
The light-emitting device may further comprise: a third electroluminescent element for emitting light of a third brightness when energised by the predetermined potential, the third brightness being in between that of the first and second brightness; and means for energising the third electroluminescent element at the predetermined potential. The third electroluminescent element may have a light-emitting area greater than that of the first element and less than that of the second element.
There may be provided a graphic display comprising a light-emitting device according to the second aspect of the invention, wherein each electroluminescent element corresponds to a pixel for displaying graphic information.
In accordance with a third aspect of the invention, there is provided a light-emitting device comprising: a first electroluminescent element for emitting light of a first colour when energized; and a second electroluminescent element for emitting light of a second colour when energized, the first electroluminescent element emitting light at a lower luminance than that of the second electroluminescent element when each is energized with optimal efficiency; characterized in that the first and second electroluminescent elements are configured to operate at the same perceived brightness when both electroluminescent elements are energized with optimal efficiency.
The third aspect of the invention provides for operating light emitting elementsxe2x80x94having different luminance versus efficiency characteristicsxe2x80x94at optimal efficiency whilst at the same time achieving uniformity in the light output from each element. Such a light emitting device may be valuable where lifetime is not an issue, which may well be the case in certain xe2x80x9cthrowawayxe2x80x9d applications.
The first or second electroluminescent elements may comprise an organic light-emissive material. The organic light-emissive material may be polymeric. The first and second electroluminescent elements may be energized by a common potential.
The first and second electroluminescent elements may be energized by different potentials, and the second electroluminescent element may be energized at a higher potential than the first electroluminescent element. The energizing potential applied to one or both of the electroluminescent elements may be pulsed, with the first electroluminescent element perhaps being energized for longer periods than the second electroluminescent element. For example, the first electroluminescent element may be pulsed more frequently or with a greater pulse width than the second electroluminescent element.
The first electroluminescent element may be configured to emit light over a larger area than the second electroluminescent element. The ratio between the light emitting areas of the first and second electroluminescent elements (A1 and A2) may be substantially equal to the ratio between the luminances of the second and first electroluminescent elements (L2 and L1) respectively when energized at optimal efficiency, i.e., A1/A2 L2/L1.
The first and second colours of light emitted from the electroluminescent elements are selected from the group consisting of red, green and blue. The device may further comprise a third electroluminescent element for emitting light of a third colour when energized, the third electroluminescent element emitting light at a luminance in between that of the first and second electroluminescent elements when energized with optimal efficiency.
There may also be provided a graphic display comprising a light-emitting device according to the third aspect of the invention, wherein each electroluminescent element corresponds to a pixel for displaying graphic information.
In accordance with a fourth aspect of the invention, there is provided a light-emitting device comprising: a first electroluminescent element for emitting light of a first colour when energized; and a second electroluminescent element for emitting light of a second colour when energized, the first electroluminescent element having a half-life which is greater than that of the second electroluminescent element when each is energized with optimal efficiency; characterized in that the second electroluminescent element is configured to operate at a lower brightness than the first electroluminescent element when both are energized with optimal efficiency.
The third aspect of the invention provides for operating light-emitting elementsxe2x80x94having different half-life versus efficiency characteristicsxe2x80x94at optimal efficiency and in such a way that the service life of the device is increased.
The first or second electroluminescent elements may comprise an organic light-emissive material. The organic light-emissive material may be polymeric. The first and second electroluminescent elements may be energized by a common potential.
The first and second electroluminescent elements may be energized by different potentials, and the second electroluminescent element may be energized at a higher potential than the first electroluminescent element. The energizing potential applied to one or both of the electroluminescent elements may be pulsed with the first electroluminescent element perhaps being energized for longer periods than the second electroluminescent element. For example, the second electroluminescent element may only be energized for short periods whilst the first electroluminescent element is operated continuously. The ratio between the energization times for the first and second electroluminescent elements (t1 and t2) may be substantially equal to the ratio between the half-lives of the second and first electroluminescent elements (T2 and T1) respectively when energized at optimal efficiency, i.e. t1/t2≈T2/T1.
The first electroluminescent element may be configured to emit light over a larger area than the second electroluminescent element. The ratio between the light emitting areas of the first and second electroluminescent elements (A1 and A2) may be substantially equal to the ratio between the brightnesses of the second and first electroluminescent elements (B2 and B1) respectively when energized at optimal efficiency, i.e., A1/A2 B2/B1.
The first and second colours of light emitted from the electroluminescent elements are selected from the group consisting of red, green and blue. The device may further comprise a third electroluminescent element for emitting light of a third colour when energized, the third electroluminescent element emitting light at a luminance in between that of the first and second electroluminescent elements when energized with optimal efficiency.
There may also be provided a graphic display comprising a light-emitting device according to the third aspect of the invention, wherein each electroluminescent element corresponds to a pixel for displaying graphic information.